lbridges, my apologies for not addressing the point you raised, but I missed your post. You raise a good point. It wasn't my intention to imply this work was conclusive. Frankly I think it is far from perfect for a number of reasons and you have added another. Rather I posted it because it at least raises an interesting question, do we know if guidelines that are so widely accepted are correct? Even more important they suggest what sounds like a reasonable method for determining how long a filter needs to run (I think we already have an answer for the sanitizer). Test the return water. If it is as clean as the suction water, there is no reason to be running the filter. There might be a better or more complete method, but its a start. Pool owners spend a lot of time testing the water to make sure water parameters, including sanitization is where it should be and for good reason, the consequences of failing to do so are dramatic and potentially very costly. Yet we spend very little time thinking about filtering, or at least over filtering. Again with good reason, the consequences are less obvious and onerous (a high electric bill). But that doesn't mean its impossible, quite the reverse. Dismissing the study as imperfect misses the point. Calling a pool a luxury in order to justify poor design makes no sense. A more reasonable response should be to say, interesting data, we should try and learn more and see if it possible to rethink how we operate our pool equipment in a way that maintains water quality yet consumes less energy. It is a noble goal and one that appears likely to pay off. Two of the biggest advances towards this end would be ECM motor powered infinitely variable speed pumps, like the Intelliflo and venturi skimmers. Where previously pumps had to be sized for the maximum flow (like when the heater was going) flow rates can be set for whatever function is currently being called for. This is a massive change. As for venturi driven skimmers where previously it required pumping 40 gpm through the suction line, all the piping, filter, pump, heater with their respective losses (and energy consumption) to achieve 40 gpm through the basket, these skimmers require only 8 gpm to move the same 40 gpm of water. That is a big deal. Even without reduced turnover times just these two changes can make an enormous change in the energy use of a pool. The possibility of lower run times on top of these lower flow rates when running, merely add to the energy saving opportunity. This is not esoteric stuff. Pumps die and need to be swapped out and there have been numerous discussions on this forum asking what the expected energy savings are of replacing a single speed pump with an Intelliflo (or a two speed pump). The answer if you just swap out but program as before is not much. The real gain and its big, is to use the capability of your new pump to run at any speed to run at the right speed for each task and to only run as long as is necessary. Luckily this is easy to do. As best I can tell the resistance to do so is mostly due to a lack of understanding of how to calculate the proper flow and turnover rates. Not wanting to make a mistake that will come back to bite them and unwilling to admit they don't understand how to calculate the proper answer its easier to put out vague assertions about why its bad or will cause problems. I'm an engineer. I don't think its scary when you don't know something, its a challenge. Either you poke around until you find someone who does (hence my posts here) or if it turns out nobody knows you propose a method for figuring it out, put it out there and see if it appears sound (which was why I posted the article and keep trying to get a fruitful discussion going). As for me I'm in the final stages of installing a pool with venturi skimmers (long in use in commercial pools), carefully sized pumping, gentle curves and a low back pressure filter and MPV (off the shelf from Pentair, nothing special). The robot will be a pressure driven 380 but because the pump is variable speed I won't need a boost pump (saved $$ and much more efficient due to the Intelliflo's ECM motor). The whole system will run at 16 gpm when filtering, which given the head will result in power consumption of less than a quarter of a standard single speed pump. It pops up to 40 gpm when heat exchanger (I'm using hot water from the same heat pumps that heats and cools the house to heat the pool) comes on, but that is rarely going to be more than 3 hours a day. Even if I ran the system 24/7 my payback for the pump is no more than a year. If it turns out running less than that is possible all the better. So please excuse the long post but if anyone is interested in this kind of thing and wants to keep bouncing ideas back and forth I'd greatly appreciate it. Problem solving is a team sport.

dlcz, Sorry if I wasn't clear. I am NOT in favor of injecting Ozone into the suction side of the pump. I suggest following manufactures recommendations for installation of an Ozone generator as I do for any piece of equipment. I was more interested in looking at this from a theoretical point of view if it was even possible. Based on Woodberg's post, it looks as though it is possible but it depends on some of the operating conditions. Initially, what I took issue with is that injected air bubbles are the same as cavitation. As I pointed out before the physics are different. So the air itself is not damaging to the impeller and should pass without incidence much like it does when the pump primes or when there is a small leak in the suction side of the pump. However, I would agree that with enough air flow will cause problems with pump efficiency and the pump could lose prime at the extreme. But this is all dependent on the amount of air introduced into the water flow. Again, Woodberg indicated this is dependent on the flow rate of the pump and the air injection rate of the unit which is logical. In my previous post, I neglected the total air flow passed through the unit which was a very good point and can be quite significant. I was in a rush and didn't think about that so good catch. From what I can tell, a typical ozone unit has a total air flow of about 15 SCFH which is a little less that 2 GPM but that is at the maximum setting. Neglecting any ozone that is dissolved, this could be a problem for low water flow rates but normally the maximum ozone setting would not be used with the minimum flow rate setting. This seems to be consistent with what Woodberg reported and means that you can adjust the settings to make it work (i.e. lower ozone production rate for lower water flow rates). In any case, reduction in flow rates should be less than what is experienced with a dirty filter so that seems to be less of an issue. Air may accumulate in the filter but most filters have air bypass mechanisms to flush out air. So let me reiterate that I am not suggesting that any one should do this only that it may be possible if absolutely necessary. _________________ However, I would like to make a point about using ozone generators in general. First, I don't think they are necessary and really don't reduce the need for chlorine very much. The important part of any sanitation system is to have a residual in the water when you need it to kill bacteria. Chlorine is the most effective way to provide a residual and ozone simply cannot do this. Water must pass the ozone unit before the bacteria is killed. This could take several hours of pump run time before the bacteria is killed. Bacteria needs to be killed at the source within seconds, not hours which requires a minimum FC level. At best, systems with Ozone might be able to operate at the same residual FC level as a SWCG (~5% of CYA level). The Ozone helps with oxidation which can help reduce the chlorine demand so you don't need to add as much each day but you still will need at lease the same residual as a SWCG. Also, many of the reasons sited for Ozone use are bit deceiving and require further examination of the examples. Most of the problems sited about chlorine stem from the improper management of indoor pools. Often times these pools do not have any CYA in the water and have fairly high chlorine levels which can have very bad side effects. This is a dangerous condition but is easily rectified with proper pool water management. Most indoor pools operate without CYA because there is no need for UV protection. However, without CYA, a very small chlorine residual of 0.1 ppm is required (based upon studies of kill rates at various chlorine levels). The problem is that it is very difficult to maintain the chlorine at this level so most indoor pools operate well above this level. This is what causes most of the problems with air quality and corrosion. Contrary to popular belief, it is a good idea to have a little CYA in an indoor pool (~10 ppm) to help buffer the chlorine so it is not so reactive. Also, chlorine is easier to maintain since a CYA of 10 ppm requires about 1 ppm of free chlorine for proper kill rates. A 1 ppm FC with a 10 ppm CYA is much less reactive than a 0.1 ppm FC level at 0 ppm CYA. With the higher CYA, the bacteria kill rate is still very fast but the reaction with metal and fabrics as well as the evaporation rate is much less. So there are much fewer problems when CYA is used. However, too much CYA for given FC level can also cause problems. Putting all the marketing hype aside, the safest pool is a properly maintained chlorine pool.

Eye friendly water comes from proper balancing and filtering. Eyes have a lot of salt. Non salt pools may draw some of the salt out but salt pools lower this tendency. Ozone oxidizes impurities such as skin oils, sun tan lotions, and a litteny of other things. Chlorine breaks them down also but may take a little longer. Pool salt is pure table salt with out anti caking materials and no iodine. It is just Sodium Chloride. Simply put, when the water with this added salt passes an energized cell, the salt molecule is split, freeing the chlorine. When water washes over tablets, they free chlorine. My point is that chlorine is chlorine. Both systems work. While chlorine is corrosive, the low quantities found in a normal pool chemistry is not sufficient to cause harm. Improper bonding can result in electrolysis, aka a stray voltage resulting in a current flow. This can cause corrosion. Improper pool chemistry can also cause corrosion or scale to form. This is a chemical change in the materials. Large temperature swings, whether rapid or gradual will not cause corrosion, but rather will cause erosion. This is a physical change in the material. Pure water, does NOT conduct electricity unless an extremely high voltage is present, such as lightning. Adding chemicals and minerals does lower the voltage required for water to become an electrolyte, that is a liquid conductor. Electricity is the the flow of electrons. It, like water, will follow the path of least resistance in proportion to the paths it has available. Bonding is method equalizing and dispersing of stray voltages that can create current flows. These are typically sent to the same ground and the main power source for the home. Having two grounds separated by some distance may be able to sink stray current in different amounts. This difference in potential may create a battery to form between them. It may not be much, but its enough to cause electrolysis. I have never seen a properly bonded pool have salt cell related corrosion. There has always been another underlying cause of the corrosion, if, in fact, it was corrosion and not erosion at work. Please also know that water is also know in the chemistry world as the Universal Solvent. Next to the sun, water is a pool's biggest threat. Water can carry many thing in solution such as salt, calcium, chlorine, silver, iron, copper and a litany of other substances. Scott

I would do a combination of both. We install a lot AOP systems for swimming pools, that also are on SWG's. With this system, it allows the chlorine level produced by the SWG, to be kept at a lower level, yet still keep the water sanitized. Del's AOP system is what we use. It utilizes Ozone and Ultraviolet together to kill bacteria. Like any other pool/spa products, it's not a set it and forget it. Water must still be tested and adjusted regularly. Especially a spa, as it's such a small body of water and can change very quickly. https://www.c-m-p.com/spa-products/spa-sanitizers/del-aop-spa/